When articles are made of plastic on blow molding machines, the plastic is substantially stronger if the plastic material has been stretched in two directions at right angles to one another. This results in "bi-axial orientation". With some plastics bi-axial orientation is necessary in order to obtain a clear transparent article. With other materials the bi-axial orientation substantially increases the strength of the final product even though no clear transparency can be obtained.
This invention provides improved apparatus and method for making bi-axially oriented products by blow molding. In order to obtain more flexibility in handling the parison on the core rod, this invention uses a core rod which is covered with an elastic balloon. This balloon hugs the core rod when the balloon is deflated and the parison is applied over the surface of the balloon in the injection mold.
The core rod is transferred from the injection mold to a conditioning station where the plastic of the parison is brought to orientation temperature; that is, the temperature at which solidification begins. The parison is then stretched in the direction of its length by increasing the length of the core rod and stretching the balloon lengthwise of the core rod with the plastic material coated over the balloon.
This stretching operation is performed in a mold with special provision for permitting movement of the plastic in the mold lengthwise of the core rod and with no contact, or only light contact, between the plastic and the mold surfaces.
Following this initial stretching, the core rod and parison are moved to a second conditioning station and then to a blowing mold in which the parison is preferably blown to its final shape.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.